Drive feedthrough nulling system

ABSTRACT

The present invention relates to a system for nulling drive feedthrough error in a sensor having first and second drive electrodes which impart vibratory motion to first and second proof masses in response to first and second opposite phase drive signals, and having first and second capacitances defined between the drive electrodes and their associated proof masses. A mismatch between the first and the second capacitance is measured. Drive feedthrough caused by the measured capacitance mismatch is nulled by adjusting the relative amplitudes of the first and second opposite phase drive signals, whereby the ratio of the amplitudes is proportional to the ratio of the first and second capacitances. A servo loop may adaptively effect the ratio of amplitudes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 09/631,249, entitled DRIVE FEEDTHROUGH NULLING SYSTEM and filedAug. 2, 2000 now U.S. Pat. No. 6,445,195.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable

REFERENCE TO MICROFICHE APPENDIX

Not Applicable

FIELD OF THE INVENTION

This invention relates generally to feedthrough nulling systems, andmore particularly to systems for nulling drive signal feedthrough insensors.

BACKGROUND OF THE INVENTION

Coriolis force sensors such as tuning-fork gyroscopes are known in theart. These sensors can be fabricated using MEMS (microelectromechanicalsystems) techniques, by way of example. Micromachined Coriolis forcesensors generally include a pair of proof masses which are positioned onopposite sides of an input axis, and which are connected to a substrateby support flexures. The flexures support the proof masses so as toallow the proof masses to vibrate in a nominal plane that includes theinput axis. Time varying drive signals are provided by way of driveelectrodes to impart an in-plane, anti-parallel vibratory motion to theproof masses. Typically, the frequency of the drive signals is set at ornear the mechanical resonant frequency of the sensor assembly.

Position sensitive in-plane pick-off electrodes are usually provided forsensing in-plane displacements of the proof masses caused by thevibratory motion. The pick-off electrodes are coupled to a feedback gaincontrol circuit, which controls the amplitude of the drive signals, andthus the amplitude of vibration of the proof masses. For stable andpredictable gyroscope performance, the amplitude of vibration of theproof masses is preferably maintained at a predetermined, constantlevel.

Upon rotation of the sensor assembly about the input axis, the vibratingproof masses produce anti-parallel, out-of-plane deflections, caused byCoriolis forces which arise in response to the rotation. Out-of-planesensing electrodes are provided in order to detect the out-of-planedeflections. Due to the nature of the Coriolis force, whose magnitudedepends on the rotational rate of the sensor assembly, the amplitudes ofthe Coriolis deflections are proportional to the rotational rate of thesensor about the input axis. The sensing electrodes thus generate asignal indicative of the rotational rate of the gyroscope.

The performance of sensors such as the tuning fork gyroscope discussedabove is adversely affected by a feedthrough of the drive signals intothe sensor output, resulting from a coupling of signals from the driveelectrodes into the out-of-plane sensing electrodes. Drive feedthroughaffects measurements by both the out-of-plane sensing electrodes and thein-plane pick-off electrodes, thus limiting gyroscope performance.

It is an object of this invention to provide a system for significantlyreducing drive feedthrough, thereby improving sensor performance.

SUMMARY OF THE INVENTION

The present invention relates to a system for nulling drive feedthrougherror in sensors such as tuning fork gyroscopes. By reducing theundesirable effects of drive feedthrough, the present invention reducesone of the largest electronic sources of error in tuning forkgyroscopes. Further, the technique of the present invention has generalapplicability, and can be used to null other forms of drive feedthrough.

In one embodiment, the present invention relates to a system for nullingfeedthrough error in a sensor having first and second drive electrodeswhich impart vibratory motion to first and second proof masses inresponse to first and second opposite phase drive signals, and havingcapacitances defined between the drive electrodes and their associatedproof masses. The present invention is predicated in part on therecognition that capacitance mismatch between the drive electrodes andthe associated proof masses is a dominant cause of drive feedthrough inCoriolis force sensors.

In accordance with the present invention, a differential drive amplitudeis adjusted in order to compensate for drive capacitance mismatch.Compensation is accomplished by measuring the capacitance mismatchbetween the first and second drive electrodes and their associated proofmasses, and nulling the measured capacitance mismatch by adjusting thedrive signals. The capacitance mismatch can be measured by detectingsignals that are proportional to the capacitance mismatch. In oneembodiment, an off-frequency artifact is detected in order to obtain ameasure of the drive capacitance mismatch.

In accordance with the present invention, a sensor for detecting arotational rate about an input axis includes a substrate, and first andsecond proof masses positioned on opposite sides of an input axis. Theproof masses are flexurally coupled to the substrate so as to permit anin-plane vibratory motion of the masses in a nominal plane including theinput axis, and an out-of-plane vibratory motion in a directionperpendicular to the nominal plane. Each proof mass includes aelectronically conductive region.

A driver includes first and second drive electrodes opposite theelectrically conductive regions of the first and second proof masses.First and second capacitances are defined between the first driveelectrode and the first proof mass, and between the second driveelectrode and the second proof mass, respectively. The first and seconddrive electrodes are adapted to receive first and second a.c. oppositephase electrical drive signals for electrostatically driving the firstand second proof masses to effect the anti-parallel vibration in thenominal plane. The frequency of the drive signals is preferably set tobe substantially equal to a fundamental frequency corresponding to amechanical resonant frequency of the sensor.

First and second sensing electrodes are provided for detecting the outof plane vibratory motion perpendicular to the nominal plane, arisingfrom the Coriolis force that acts upon the proof masses when the sensorassembly rotates about the input axis. Each sensing electrode is fixedlypositioned with respect to the substrate, and is disposed opposite theconductive region on a respective one of the first and second proofmasses. The sensor is preferably micromachined, i.e. the proof masses,the drive electrodes, and the sensing electrodes are made from an etchedsilicon structure.

A circuit generates an output signal corresponding to the separationbetween the sensing electrodes and the conductive regions of the proofmasses opposite. The output signal is representative of the rotationalrate of the sensor about the input axis. Means are provided foradjusting the relative amplitudes of the first and second a.c. drivesignals, whereby the ratio of the amplitudes is proportional to theratio of the first and second capacitances. The adjustment of relativeamplitudes of the drive signals substantially nulls drive feedthrough inthe output signal caused by a mismatch in the first and secondcapacitances. In particular, when the drive signals have oppositepolarities, and have a magnitude ratio equal to the reciprocal of theratio of the corresponding capacitances, drive feedthrough from drivecapacitance mismatch is substantially nulled.

The circuit may include first and second circuits. The first circuit isoperative to provide an indicative signal proportional to a capacitancemismatch between the first and second capacitances. The second circuitis operative to measure the capacitance mismatch using the indicativesignal, and to compensate for the measured capacitance mismatch byadjusting the first and second drive signals so as to substantially nulldrive feedthrough.

The first circuit may include a sense preamplifier connected to theproof masses. The output from the sense preamplifier contains a signalthat is indicative of the mismatch between the first and secondcapacitances. The indicative signal can be demodulated in the secondcircuit to obtain a measure of the drive capacitance mismatch. Theindicative signal may be a separate tracer signal that has been added tothe drive signals. Alternatively, the indicative signal may be aninherent off-frequency component of the drive signal, in which thesensor resonant frequency and its harmonics have been suppressed.

The second circuit may include an adjustment device responsive to acontroller for effecting the ratio of amplitudes, and a servo loop foradaptively effecting the ratio of amplitudes. In one embodiment, theadjustment device includes first and second variable gain amplifiers(VGAs) interposed between the servo loop and the first and second driveelectrodes. The VGAs are operative to adjust the differential amplitudeof the drive signals in response to the controller.

The second circuit may include means for demodulating the indicativesignal from the first circuit so as to obtain a DC signal proportionalto the capacitance mismatch. In one embodiment, the means fordemodulating the indicative signal may include a multiplier, a low passfilter for removing modulation components from the indicative signal,and a controller. The demodulated signal may be passed through thecontroller to provide a control signal. The control signal may be splitinto first and second control signals of opposite phase, which controlthe first and second VGAs, respectively.

The present invention also relates to a method for nulling drivefeedthrough in a sensor having first and second drive electrodes whichimpart vibratory motion to first and second members in response to firstand second drive signals, and having first and second capacitancesdefined between the first and second drive electrodes and theirassociated proof masses. A mismatch between the first capacitance andthe second capacitance is measured. Drive feedthrough caused by themismatch is nulled by compensating for the measured capacitancemismatch. The relative amplitudes of the first and second drive signalsis adjusted in order to compensate for the measured capacitancemismatch.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the present invention will be apparentfrom the following detailed description of the drawing in which:

FIG. 1 is a perspective view of a prior art Coriolis sensor.

FIGS. 2(a) and 2(b) illustrate a drive coupling model for the sensor ofFIG. 1.

FIG. 3 is a diagram of an automatic drive feedthrough nulling system, inaccordance with the present invention, for the sensor of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a prior art sensor 10, such as a micromechanicaltuning-fork gyroscope, for detecting a rotational rate about an inputaxis 11 of the sensor 10. The sensor 10 includes a substrate 32, andfirst 20 and second 22 proof masses positioned on opposite sides of theinput axis 11. The proof masses 20 and 22 are flexurally coupled to thesubstrate 32 by means of support flexures 30. In one embodiment, theproof masses 20 and 22 are suspended by the flexures 30 above thesubstrate 32, so as to permit vibratory motion of the proof massesin-plane in a nominal plane including the input axis, as well asout-of-plane in a direction perpendicular to the nominal plane. Thenominal plane may be parallel to the surface of the substrate 32. Eachproof mass includes an electrically conductive region, shown as 21 a and21 b, respectively.

The prior art sensor 10 includes a driver having first 14 and second 16drive electrodes disposed opposite the electrically conductive regions21 a, 21 b of the first 20 and second 22 proof masses. The sensor 10also includes first 24 and second 26 out-of-plane sensing electrodes,and an in-plane position-sensitive pickoff electrode 28. In oneembodiment, the in-plane pickoff electrode 28 is also suspended abovethe substrate 32 by the flexures 30, while the out-of-plane sensingelectrodes 24 and 26 are disposed on the substrate 32.

In one embodiment, each proof mass has comb electrodes 18 extendinglaterally therefrom and toward an adjacent one of the drive electrodes.The drive electrodes 14, 16 have complementary comb electrodes 18extending toward, and interleaved with, the comb electrodes 18 of theadjacent one of the proof masses 20 and 22. The drive electrodes 14 and16 are electrostatically coupled to the proof masses 20 and 22,respectively, by the sets of interleaved comb electrodes 18. First andsecond capacitances C₁ and C₂ are thus defined between the first 14 andsecond 16 drive electrodes, and the first 20 and second 22 proof masses,respectively.

The first and second drive electrodes 14 and 16 are adapted to receivefirst and second a.c. opposite phase electrical drive signals whichinduce electrostatic forces between the sets of interleaved combelectrodes 18 between electrode 14 and proof mass 20 on the one hand andelectrode 16 and proof mass 22 on the other. The drive signals typicallyare square waves. The drive electrodes convert the drive signal into anelectrostatic Coulomb force signal proportional to the square of thedrive signal, thereby electrostatically driving the proof masses toimpart in-plane vibration to the proof masses in the nominal plane (inthe directions shown by arrows 15).

In response to a rotation of the sensor 10 about the input axis 11, theproof masses deflect out of the plane of vibration. Such out-of-planedeflections of the proof masses 20, 22 occur because of the Coriolisforces that arise from the rotation around the input axis and act on themoving proof masses. The Coriolis force is given by:

{right arrow over (F)}=−2m{right arrow over (Ω)}×{right arrow over(V)}  Eq. 1

where

m=mass of the proof mass,

{right arrow over (Ω)}=angular velocity of the proof mass about theinput axis,

{right arrow over (V)}=velocity of in-plane vibration of the proof mass,and

x denotes the vector cross-product.

Since the in-plane vibrations occur at a drive frequency, from Eq. 1 itfollows that the out-of-plane deflections of the proof masses also occurat the drive frequency. The drive frequency is typically set to besubstantially equal to the mechanical resonant frequency of the sensor,generally referred to as the fundamental frequency.

In order to maintain stable gyroscope performance, the vibratoryamplitudes of the proof masses 20, 22 should be kept relativelyconstant. This is accomplished by monitoring the in-plane proof massdeflection with the position sensitive pick-off electrode 28. Thepick-off electrode 28 produces an output signal which is indicative ofthe amplitude of the in-plane motion of the proof masses. The outputsignal from the pick-off electrode 28 is fed to a feedback gain controlloop to control the amplitudes of the drive signals, and thus theamplitudes of the vibratory motion of the proof masses. The detection ofin-plane deflection is susceptible, however, to errors from drivefeedthrough.

First and second sensing electrodes 24, 26 are disposed on the substrate32 for detecting the anti-parallel, out-of-plane Coriolis deflections ofthe proof masses 20 and 22. The amplitudes of the out-of-planedeflections are proportional to the input rotational rate, according toEq. 1. The sensor output signals therefore provide a measure of therotational rate {right arrow over (Ω)}.

In one embodiment, capacitive sensing is used. In this embodiment, eachout-of-plane sensing electrode is fixedly positioned with respect to thesubstrate 32, and is disposed below a conductive region (21 a or 21 b)of a respective one of the first 20 and second 22 proof masses. Thecapacitances between the sensing electrodes 24, 26 and the proof masses20, 22 thus oscillate in accordance with the amplitudes of theout-of-plane Coriolis deflections. The sensing electrodes generateoutput signals that are proportional to the separation between theconductive regions 21 a, 21 b, and the associated sensing electrodes 24,26, and hence are proportional to the amplitudes of the Coriolisdeflections of the proof masses. The sensor output signals thus reflectthe changes in the capacitances between the sensing electrodes and theirassociated proof masses, and vary according to the frequency of theout-of-plane oscillations.

A DC bias voltage V_(b) is applied to the sensing electrodes 24, 26 toestablish a potential difference. Because of the capacitances betweenthe conductive regions 21 a, 21 b and the associated sensing electrodes,the sensing electrodes 24, 26 induce on each proof mass a charge that isproportional to the separation between the proof mass and its associatedsense electrode. The changes in capacitance between the proof masses 20,22 and their associated sensing electrodes 24, 26 thus result in changesin the charge on the proof masses 20, and 22, which are reflected in thesensor output signals.

The detection of the out-of-plane deflections is susceptible, however,to errors resulting from drive feedthrough into the sensor outputsignal. In particular, the sensor output signal is contaminated bycapacitive coupling from the drive electrodes. Such capacitive couplingis caused by parasitic capacitances related to the fabrication andpackaging of the sensor system. Drive feedthrough includes synchronousand asynchronous components. Synchronous drive feedthrough includessignals at the fundamental frequency and its harmonics. Asynchronousdrive feedthrough includes all other signals originating at the driveelectrodes. Synchronous feedthrough causes a direct instrument bias.Asynchronous feedthrough imposes gain limitations and limitsdesensitization of the sensor to DC offsets.

An off-frequency drive scheme for eliminating synchronous feedthrough isdisclosed in prior art U.S. Pat. Nos. 5,481,914 and 5,703,292, both ofwhich are incorporated herein by reference. In this scheme, the sensor10 is connected in a feedback relationship with a frequency translationcircuit. The input drive signal is converted into a feedback signalwhich is further processed to control the drive signal in a closed loop.The frequency translation circuit suppresses components of the feedbacksignal at the resonant frequency of the sensor, by effectivelymultiplying the feedback signal by a commutation signal. Any coupling ofthe drive signal into the sensor output signal is then readily removedby using conventional filtering techniques, since the drive signal nowhas a frequency different from the frequency of the sensor outputsignal. The frequency translation circuit thus substantially reducessynchronous feedthrough, i.e. in-band coupling of the drive signal intothe feedback signal and into the sensor output signal. The off-frequencydrive scheme summarized above still does not remove, however,asynchronous feedthrough.

The present invention provides a system for substantially reducing bothsynchronous and asynchronous feedthrough. It has been found that thedominant cause of drive feedthrough is a mismatch in the capacitances C₁and C₂ between the drive electrodes 14, 16 and their associated proofmasses 20, 22. The difference between C₁ and C₂, which should ideally bezero, is the main cause of drive feedthrough. The present inventioncompensates for variations in C₁ and C₂ by varying the drive voltages V₁and V₂ so as to equalize the magnitudes of the charges Q₁ and Q₂ inducedon the first and second proof masses by way of the drive voltages. Thecharges are related to the drive voltages by the relation Q=CV, where Cis the associated capacitance, C₁ or C₂. By balancing out the chargesinduced on the proof masses, i.e. by rendering the charges equal andopposite, feedthrough of the drive signals into the sensor outputsignals is nulled. In the present invention, compensation forcapacitance mismatch is accomplished by measuring one or more of thedrive feedthrough components, and then nulling the measured componentsby adjusting the relative amplitudes of the drive signals V₁, V₂, asexplained further below.

FIGS. 2(a) and 2(b) provides a circuit diagram of a drive coupling modelfor the sensor discussed above. In FIG. 2(a), a sense preamplifier 31 isshown connected to the proof masses 20 and 22. The proof masses areplaced at a virtual ground of the sense preamplifier 31. A first C₁ anda second C₂ capacitance is defined between each drive electrode and thesumming node of the sense preamplifier 31.

Referring to FIG. 2(b), the preamplifier output V_(o) is given by:$\begin{matrix}{V_{o} = {{- {C_{f}^{- 1}\lbrack {{V_{1}C_{1}} + {V_{2}C_{2}}} \rbrack}} + {V_{e}\lbrack {1 + ( \frac{C_{1} + C_{2} + C_{N}}{C_{f}} )} \rbrack}}} & {{Eq}.\quad 2}\end{matrix}$

where V_(e) is the preamplifier input error voltage, which is generallysmall unless the input changes very rapidly, and the voltages andcapacitances are as shown in FIG. 2(b). If the preamplifier bandwidth ismade arbitrarily large, the error term becomes arbitrarily small, andthe preamplifier output is simplified, becoming:

V _(o) =−C _(ƒ) ⁻¹ [V ₁ C ₁ +V ₂ C ₂ ]=−C _(ƒ) ⁻¹ [Q ₁ +Q ₂]  Eq. 3

where Q₁ and Q₂ represent the charge transferred to the summing node byC₁ and C₂, respectively. Drive feedthrough in the preamplifier outputcan thus be nulled by effecting a charge balance condition:

Q ₁ =−Q ₂  Eq. 4

The charge balance condition can be alternatively expressed as:$\begin{matrix}{\frac{V_{1}}{V_{2}} = \frac{- C_{2}}{C_{1}}} & {{Eq}.\quad 5}\end{matrix}$

Therefore, if V₁ and V₂ have opposite polarities and a magnitude ratioequal to the reciprocal of the ratio of the corresponding capacitors,drive feedthrough fromstatic capacitance mismatch is nulled.

In the present invention, drive feedthrough can be measured and nulledin one of several embodiments. In order to measure drive feedthrough, asignal that is proportional to the drive capacitance mismatch must firstbe obtained. In one embodiment, a separate trace signal having acharacteristic such as frequency different from the drive signals, canbe obtained from a trace source and added to the drive signals. Thetrace signal is separately detected, and used to measure drivecapacitance mismatch. In another embodiment in which the drive signalinherently contains components at off-frequencies, as in theoff-frequency drive scheme discussed above, one or more of the inherentoff-frequency components of the drive signal can be detected that areproportional to the capacitance mismatch. The detected off-frequencycomponents are then demodulated in order to measure the capacitancemismatch. The measured capacitance mismatch is nulled by making a fixedadjustment to the differential drive voltages, or with an automaticfeedback servo loop.

FIG. 3 illustrates one embodiment of the present invention in which anoff-frequency artifact of the drive signal, i.e. an inherentoff-frequency component of the drive signal, is used to measure thecapacitance mismatch. A nulling compensator 36 is used in conjunctionwith a feedback servo loop 38, to compensate for the measuredcapacitance mismatch. The nulling compensator 36 demodulates theoff-frequency artifact to provide a measure of capacitance mismatch.Based on the measured capacitance mismatch, the differential amplitudeof the drive signals V₁, V₂ is modified so as to effect the chargebalance condition of Eq. 5, thereby substantially reducing drivefeedthrough into the sensor output signal.

The capacitance mismatch between the drive electrodes 14, 16 and theirassociated proof masses 20, 22 is measured by detecting an off-frequencyartifact of the drive signals V₁, V₂ at the output of the sensepreamplifier 31, and demodulating the detected artifact. In oneembodiment of the present invention, the form of the drive signals isdescribed by the following equation: $\begin{matrix}{{{V_{D}(t)} = {( {B + {A\quad s\quad {q( {\omega \quad t} )}}} )s\quad {q( \frac{\omega \quad t}{n} )}}},{n\quad e\quad v\quad e\quad n}} & {{Eq}.\quad 6}\end{matrix}$

where ω is the fundamental frequency, and sq(ωt) is a zero-meansquare-wave with amplitude=1 volt and period 2π/107.

Because of mismatch in the time-average drive capacitance, a smallerversion of the signal of Eq. 5 exists, along with other signals, at thepreamplifier output. That is, the amplified preamplifier output is:$\begin{matrix}{{{V_{o}(t)} \approx {{{K( {B + {A\quad s\quad {q( {\omega \quad t} )}}} )}s\quad {q( \frac{\omega \quad t}{n} )}} + {o\quad t\quad h\quad e\quad r\quad s\quad i\quad g\quad n\quad a\quad l\quad s}}},} & {{Eq}.\quad 7}\end{matrix}$

where the value of K is given by: $\begin{matrix}{K = {G_{A\quad C}( \frac{\Delta \quad C_{D}}{C_{f}} )}} & {{Eq}.\quad 8}\end{matrix}$

in which G_(AC) is the gain following the preamplification, and ΔC_(D)is the mismatch in the time-average drive capacitors.

The signal of Eq. 6 includes a significant term at frequency ω/n. Thisterm can be synchronously demodulated to yield a DC signal which isproportional to K, and thus proportional to the capacitance mismatch.The sense preamplifier output is multiplied by a square wave 93 offrequency ω/n, in order to eliminate even harmonics of the basicsubharmonic frequency ω/n. The multiplied output is given by:$\begin{matrix}{{V_{m}(t)} = {{{K( {B + {A\quad s\quad {q( {\omega \quad t} )}}} )}s\quad {q^{2}( \frac{\omega \quad t}{n} )}} = {K( {B + {A\quad s\quad {q( {\omega \quad t} )}}} )}}} & {{Eq}.\quad 9}\end{matrix}$

The modulation components can be filtered out by passing the signal ofEq. 9 through the low pass filter 52. The result is:

V _(LPF)(t)=KB=G _(AC)(ΔC_(D) /C _(f))B  Eq. 10

which is the desired DC signal that provides a measure of thecapacitance mismatch. The capacitance mismatch is thus given by:

ΔC _(D)=(C _(f) V _(LPF)(t)/B G _(AC))  Eq. 11

In the illustrated embodiment, the feedthrough nulling compensator 36 isused in conjunction with a feedback gain control loop 38. The feedbackgain control loop is operative to receive a signal 90 from the positionsensitive pick off 28. The signal 90 is indicative of the amplitude ofthe in-plane displacement of the first 20 and the second 22 proofmasses. The signal 90 is fed into a frequency translation circuit 48,which suppresses components at the fundamental frequency of the sensor10. The frequency translation circuit outputs drive signals 92 a and 92b for driving the proof masses of the sensor 10.

In one embodiment, the feedback control loop 38 includes a motorpreamplifier 42, a shifter 44, a summer 46, and the frequencytranslation circuit 48. In one embodiment, the signal 90 is fed to themotor preamplifier 42, the output of which is shifted by ninety degreesin the shifter 44. The output signal from the shifter 44 is summed inthe summer 46 with an amplitude control voltage 91. The output signal 77from the summer 46 is fed to the frequency translation circuit, andmultiplied by a commutation clock signal 79 in order to suppresscomponents in the signal 77 at the resonant frequency of the sensor 10.The output 99 from the frequency translation circuit 48 is then splitinto first and second opposite phase drive signals 92 a and 92 b fordriving the proof masses 20 and 22.

The output from the LPF 52 (Eq. 10), which is proportional to thecapacitance mismatch, is integrated in the integral controller 54. Theoutput signal from the integrated controller 54 is split into twosignals, one of which is inverted in the amplifier 56. The split signalsprovide opposite-phase control signals 94 a and 94 b, which are used tocontrol the VGAs 58 a and 58 b respectively, thus closing the automaticfeedback servo loop. The common-mode gain of the VGAs, controlled by thedrive signals 92 a and 92 b from the frequency translation circuit 48,remains constant.

The differential gain of the VGAs 58 a and 58 b is adjusted by thecontrol signals 94 a and 94 b according to the measured capacitancemismatch. In response to the control signals 94 a and 94 b, the VGAs 58a and 58 b adjust the ratio of the amplitudes of the first and seconddrive signals to be equal to the reciprocal of the ratio of the first C₁and second C₂ capacitances. The charge balance condition of Eq. 4 andEq. 5 is thus achieved, so that drive feedthrough in the sensepreamplifier output is automatically and continuously nulled. Themagnitude adjusted drive signals V₁ and V₂, shown in FIG. 3, aredelivered to the drive electrodes 14 and 16 through the VGAs 58 a and 58b.

It will be noted that the techniques described above have generalapplicability. Many forms of drive feedthrough can be nulled using thetechniques described above. It should therefore be understood that theinvention is not limited to the particular embodiment shown anddescribed herein, and that various changes and modifications may be madewithout departing from the spirit and scope of this concept as definedby the following claims.

What is claimed is:
 1. A system for nulling feedthrough error in theoutput of a sensor caused by feedthrough of one or more drive signalsinto said sensor output, the system comprising: (a) a source of a drivesignal, and a further signal having characteristics different from thedrive signal, the drive signal and the further signal being applied todrive the sensor, wherein the further signal can be detected separatelyfrom the drive signal; (b) means for detecting a response of the sensorto said drive signal and said further signal applied thereto; and (c)means, responsive to components of the further signal in the detectedresponse, for adjusting the drive signal so as to diminish thefeedthrough error.
 2. A system according to claim 1, wherein said meansfor adjusting the drive signal is responsive to a controller.
 3. Asystem according to claim 2, wherein said means for adjusting the drivesignal includes a feedback servo loop.
 4. A system according to claim 1,wherein said sensor includes at least two drive electrodes havingassociated drive capacitances and being adapted to receive equal andopposite-phase drive signals.
 5. A system according to claim 4, whereinsaid feedthrough error is caused at least in part by a mismatch betweensaid at least two drive capacitances.
 6. A system according to claim 5,wherein said further signal provides a measure of said capacitancemismatch.